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I've read somewhere that using Windex as a thinner is good with airbrushes, does this same principle apply to brushwork?
I've also read that adding a drop of washing up liquid in your water and mixing that with your paint can work as a flow aid, has anyone tried this?

I don't recommend the former (windshield washer/wiper fluid is in the same camp), primarily because, as glass cleaners, they're meant to evaporate quickly. In an airbrush, you'll likely notice significantly increased tip-dry vs. diluting with plain water, let alone proper airbrush medium or DIY mixes that utilize drying retarders. Some also warn against the potential corrosion of certain metals and the drying/cracking of synthetic materials, especially when the products contain ammonia. The same core issue (rapid evaporation) would apply to brushwork, as well. Plain old water works just fine and purpose-specific mediums work even better - considering the reasonably low cost, relative to volume, of better options, I think you're barking up the wrong tree.

The washing up liquid/dish soap thing is spot on, though. The detergent acts as a surfactant, breaking surface tension. In the sink, this helps lift grease off of your dishes. Added to paint, it improves flow. Just use it sparingly and don't be overly vigorous in your mixing - overdoing it will result in a paint well/cup full of colored froth, instead of a pool of smooth-flowing paint.

Sweet. Does the washing up liquid have any negative effects after the paints all dried up?

You can use a bit of detergent to break the surface tension. It's mostly used for washes and folks have been doing it for a long time. It's called the "wet-water" method and has been used for a long time. I don't recommend adding it to the bottle of paint, though.

I used to keep a bottle of wet-water at my paint station. I don't recall exactly, but I think I used a drop of soap per 4 oz of water. Search about for "wet water" and you should find advice from folks more qualified than myself.

No negative effects, as far as I know (experimented a bit, read lots). The amount of soap present is pretty small, in the end - you're adding a drop or two to a much larger volume of water, which, to pick an average for the sake of argument, will represent ~50% of your final mix. You may prefer to use Jet Dry or some equivalent rinse agent for dishwashers, instead - it's a surfactant (keep the water from beading up and you don't get chalky deposits on dried dishes), but is more liquid than dish soap and less prone to bubbling when agitated. Most references to "wet water" I come across actually use Jet Dry, not dish soap.

Whichever you use, start small and work up if you want to avoid potential issues. If one drop in X volume of water doesn't change much, try 3/4 X, then half (alternatively, add two drops, then three). Better than trying to paint a model with colored soap and having to strip it, along with dialing back your mix.