I also know from the Codex and Wiki that the size of regiments varies wildly between ~ 500 and 100.000, the latter being relativly rare (the example is a Valhallan infantry regiment). The size is choosen in a way to have more or less equal fighting power between the regiments. Last but not least I read that the tithe is usually expected to contain ~ 10% of the PDF forces of a planet.
The problem with 40k
here is scale. The regiment sizes are ridiculously small for the amount of soldiers they can realistically draft. Here's an example:
A single civilized world with a population of 10 billion. It has a PDF size equivalent to 1% of its total population. That's 100 million soldiers in the PDF. The Imperium come along and demand their tithe. Then the PDF give them the top 10% of their soldiery.
That's 10 million soldiers
just drafted into the Guard. There's black library stories with populations parading and celebrating regiments of a few thousand soldiers as if that's some major feat.
Now consider these 2 things as well. That 1% number is based off our current Earths percentage of armed forces to population, not the highly militaristic Imperiums so that number could be far higher. Potentially up to 10% if they have a completely focused war-economy. The second thing to consider is the ridiculousness in numbers when coming to Terra and hive worlds in general. Terra could have a tithe in the billions if its population is actually in the quadrillions, while hive worlds could have a tithe of hundreds of millions.
When it comes to things like mining or agri worlds those numbers would be far, far smaller. Though that's due to the nature of both their smaller populations and that their tithes are in resources.
1. I might be blind to the text passage, but how often do these tithes come? Is this "10% of the PDF" meant as yearly, per decade, per generation? Is it fluctuating depending on the urgency of the wartheater or the whim of the Munitorum?
Straight from the wiki:
Initial forces will be drawn from localised assets deemed sufficient for the task at hand. This includes any Guardsmen already mobilised within response range, along with regiments raised from neighbouring worlds. Such forces can be disparate in nature, requiring officers to prove themselves adept at working with whatever materiel is at hand. Should a threat escalate, or prove greater than initial response forces can handle, the Munitorum will expand their designated conflict zone, drawing down relentless waves of reinforcements to crash upon the foe until it is entirely ground to dust.
So, it really depends on need. It could be once a year, sometimes more.
2. Who decides which type of regiment is mustered (in the sense of light/heavy infantry, armored, artillery etc.)? Is this chosen by the tithed planet or by munitorum officials?
3. Who decides how big the individual regiments have to be? Is there some kind of rough overview like "Superheavy: 3 Baneblades, 5 Support tanks + Crew and technical staff, Light infantry: 4000 + 100 heavy weapon" etc. which is adjusted by the Munitorum Clerk in charge?
Typically it's decided by the Munitorum. The Munitorum dictate how many soldiers, what equipment, resources and material is required to win a war.
As for actually securing the machinery of war there doesn't appear to be a standard procedure for this one. It largely depends on the world their from and what's available. Armageddon is known for their mechanised regiments because they themselves manufacture so many Chimeras. So, the Munitorum would probably look to the world they're tithing from first to see what they can muster.
Then from there they'll start to lobby forge worlds and manufactorums to send the weapons and materials required for the manpower they've just tithed.
Regiment organisation is typically up to the planet themselves. You could have 8 - 10 man squads, 4 - 6 squads per platoon, 3 - 5 platoons per company, and 8 - 10 companies to a regiment. It's up to you.
However, super heavy companies are typically very different than other regiments. In that a super heavy regiment might only have 5 - 10 baneblades, with those vehicles either being deployed in a single mass or split up to support other units.
4. Related to that: if a regiment shows in the field that its performance deviates from the expectations, are they split/fused to adjust? I mean the case that a line infantry regiment numbering 4000 shows to be half as efficient as the usual regiment this size, are they forced to fuse with another regiment?
This happens fairly frequently. Understrength regiments are fused together to keep them at maximum strength. Preferably from the same world and culture, but that's not always the case.
Regiments are typically fought till exhaustion, if they're particularly lucky they might get the Right of Settlement. Right of Settlement allowing them to become the PDF and governors of the world they settled or reconquered.
5. Has anyone ever read in any novel what happens, when the review of troops upon the founding is deemed insufficient equipped? Would this mean the tithing fleet just loads in another 10 % of the PDF to adjust or is the planetary governor given a reprieve to muster the difference within a year or something?
I recall there being one instance of a tithe being done 9 times in one year. I can't remember if that was due to insufficient quality or quantity of troops, or just the severity of the conflict. This part is also up to you, worst case the governor would be removed or killed and replaced with someone who will get the job done.
6. Is it common to "fill up" partly depleted regiments of a world first before founding new regiments when the next tithe comes? I mean: assume half of planets regiments from the first tithe are still kicking at half their manpower, would those first be filled up to full strength or would the second tithe be purely "fresh" regiments?
It depends on distance and the frequency of tithes. If the conflict is on a neighbouring world and tithes are done regularly. Then it's highly possible. If they're being thrown into another sub-sector entirely then they'd be forced to be reshuffled into another regiment.
7. Judging from the example from the codex of a Tallarn armored regiment being reclassified as infantry regiment upon a successful attack on foot after their fuel ran out I assume this happens too? So when a regiments equipment drastically changes during deployment, they can be reclassified (either after it lost all its tanks or the other way around captured enough to change from infantry to mobile infantry)?
These can happen, though it largely depends on the Munitorum and/or what's locally available. It can work the other way as well. A regiment could say lose all their tanks and forced to fight on foot, but due to the immense bureaucracy of the Imperium could still be listed as an Armoured regiment and redeployed as such.