Are futsal sneakers okay for full-size football/soccer?

Not really a good idea, to be honest. Futsal shoes are made specifically for indoor courts - they're heavier, with a sole designed to grip smooth flooring, and on grass or artificial turf they'll slide around and put pressure on your feet. For a full-size pitch you need proper football boots or at least trainers with studs that give you decent traction with the ground. If you play occasionally, you can get away with universal football trainers, but specialized futsal shoes definitely won't work for that.

Technically you can, but I wouldn't recommend it. Futsal shoes are designed for completely different conditions - the sole grips the smooth indoor court floor, but on grass or artificial turf that grip works weird and you end up slipping. Plus, futsal shoes are usually stiffer and heavier than regular football boots, so your feet will get tired faster on a full-size pitch.

I wouldn't experiment with futsal shoes on a full-size pitch at all. The indoor court sole just isn't designed for grass or artificial turf - you'll be slipping constantly, especially when the weather's wet or the pitch has just been watered. It's not just uncomfortable, it's a injury risk: your foot can slip out when you're making a sharp stop or a burst of speed.

Beyond grip, there's also the issue of weight and how stiff the shoe is. Futsal shoes are actually heavier than regular boots, and on a full-size pitch you'll notice it after about forty minutes of play - your feet will just start aching. One match in someone else's futsal shoes was enough for me to realize it wasn't for me.

If you don't have the cash for proper boots right now, it's better to grab ground or all-purpose shoes instead - they're at least cheaper than specialized futsal footwear, and you'll be way more comfortable playing in them on the pitch. Another option: ask one of the guys to let you try their shoes and get some practice in them on a suitable semi-artificial pitch before you buy your own pair.