Yes, you can mix GF types. The concern is that the feeder Gf will be comets and they will be faster than the wider-bodied fantails and will out-compete them for food. If you stick the standard fantails, I think they'll do just fine, but if you get a GF that doesn't have a dorsal tail, like a bubble eyes or a lion head, and they might get out-competed for the food.
Note: I usually overstock my GF tanks, so if you want to be more conservative than I, that is only a good thing.
Basically, I have 1 GF per 10 gallons of water, so if I had a 125g, I would feel comfortable with 10-13 GF. I underfeed my GF, feed lots of veggies, have good filtration, and have always sworn by using a bubble wand or airstone to provide additional aeration.
I would expect that the others here would recommend 4-6 GF for a 125g.
Val