Shrimp pellets should be ok, they are normally meaty.You read right about 2", making them far too large for the tank.
Yes, Julii are schooling, just as every other Cory species is.
No, they will not do as well in a small group and in a small tank as they would have done in a large group, in a large tank.
For feeding platys, etc, twice per day as much as they can completely finish in 30 seconds, 1-2 times per week replace dry food with live (including frozen) and one day per week do not feed anything at all. For Corys, feed 1 pellet each per day until adult, then feed 1 every other day once they are adult.. what food do you have for them right now? It is well worth getting the meaty type of pellets for the Corys, not the algae type.
I bought the Omega One Shrimp Pellets because that was all they had. Sorry I didn't follow recommendations. I may get another tank when income tax season comes around, but that's a few months away. What would be the problems caused by having the Juliis, and in a group of only 4? My filter seems to be able to handle the load without any problem. I saw that I had a slight ammonia increase last night after I added them, but this morning my levels are at 0. I have a C2 filter and still have the cycled filter floss I got from PETCO. I plan on keeping that old filter floss in there at least another week or 2. It probably wouldn't be a bad idea to keep a little piece in my filter permanently right?
So as far as switching tanks, I would be able to basically break down my 10G and start up my larger tank the same day while using the same filter and wouldn't skip a beat on the cycle? Like I said, I have a Fluval C2. Would I be able to go as large as say a 29 gallon tank with that? Just move the water out of my established tank into the 29, along with my filter and plants and everything?
Mostly behaviour problems, they are usually not be as active as they normally should be.. there might also be stress problems when they spawn. Main problem would be stunted growth, which they are likely to experience even now, unless you keep doing large water changes as part of your regular maintenance.
You should be ok to upgrade like you suggest, that's what I do normally and have never had any problems that way. I also dump in a load of stem plants normally (I like my tanks planted), so if there were any problems with ammonia, the plants may just use it up. Do still test and be ready for water changes.. but as I said, I've never had any problems upgrading that way.. also, if you upgrade to a tank that big, you could always get 10 more jullii!