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So, I have some of this already, and may pick up some more if I decide to follow through.
The plan is for an all scout (mostly) 1000pt army for quick fun games. Not for tournaments, not to be expanded (already got an SM army). Just for pick up games at the FLGS.
Sure Preds aren't really scouty, but they feel like the most scouty tank the SM have.

HQ (100)
Librarian - no options

Troops (385)
10 scouts - 3 with bolt guns, 1 with Hellfire HB, Sgt Telion, then 5 with sniper rifles and camo cloaks
10 scouts - all with combat blades, sgt with PF and melta bombs

Fast Attack (310)
Landspeeder Storm with Melta (takes half of second scout squad)
Landspeeder Storm with Melta (takes other half)
Scout Bikes - 7 bikes, 2 with grenade launchers, sgt with PF and meltabombs

Heavy (205)
Predator - Twinlinked Las, Stormbolter
Predator - HvyBolter Sponsons, Dozer blade.

Plan would be HB/AC pred and LSS move up quick, Las pred, libby, 1st scout squad hang back, bikes are reactionary.

how badly will I lose?

Hordes will decimate you. Heavy armor will run you over. Inbetween armies might have some difficulty.

For the first scouts, I always go with a hellfire bolter instead. Its just funner, and high toughness Nids hate you even more.

I know you want to keep the scout theme, but if you can sqeeze it in, I would take a Vindicator over one of the preds. Much more reliable for vehicle killing, and your first turn assaults could easily tie up any threats to it for at least the first while it closes the distance.

If you drop one storm and the Las Pred a few attack bikes with MM would go great. They could count as a faster backup then pred anyway.

I think you are absolutely right on the hordes part, there aren't enough numbers in the list for them. At 1000pts though I thought I was OK on anti armor. Not great, but OK. I guess swapping the las pred for a vindi may give me slightly better anti-armor at the cost of range, but also a plate that could get dropped on hordes. Doesn't fit the fluff as well but it's not too far off.

With that though it turns the whole lot into a quick, in your face attack. Would make for some quick games.

Here's what my fluffy RG list @ 1k looks like. Its kind of an all comers list but I hope it helps.

Headquarters:
captain, power weapon = 115
command squad, meltaguns (4), razorback = 195

Troop:
Scouts (10), sniper rifles (4), shotguns (4), missile launcher = 150
Scouts (10), sniper rifles (4), shotguns (4), missile launcher = 150
Scouts (10), sniper rifles (4), shotguns (4), missile launcher = 150

Fast Attack:
Land Speeder Storm, heavy flamer = 60
Land Speeder Storm, heavy flamer = 60
Land Speeder Storm, heavy flamer = 60

Heavy Support:
Predator = 60