newish brewer question kegging question

I am not a NEW brewer per se, been doing this for about 6 or 7 months, but I am going to start brewing bigger batches now.

I just picked up 5 half barrel kegs. I know I'm going to turn one into a keggle but I'm curious if I could designate one as a primary and another as a secondary? Or do I need to find 15 gallonish glass/plastic containers for that purpose?

I can't see why I wouldn't be able to use the kegs in that way; but since I'm basically tripling the eggs in a basket I would like to be sure.

Yes, you can use kegs as fermentors. In fact, it's much better to use them since their stainless instead of plastic.

The way I do this is by removing the spear in the keg. Googling it should pull up youtube videos that explain how to simply do it.

Next, clean it out as well as you can with a hose. Once you've done this, fill it with a little bit of water. Maybe 3-4 gallons. Then (I'm assuming you have a turkey fryer if you're planning on using one as a keggle) boil the keg for a half hour or so with the spear removed. All the steam will break up any of the hard to get buildup.

Once you're done with this, soak the keg in PBW for a day or so and then sanitize before using it. If you want to get fancy, sabco sells a kit to turn these kegs into fermentors. Otherwise, a rubber carboy airlock adaptor will fit over the top of the hole with a little elbow grease.