The mini's will be fine in 1 gal pots for a few months. When the roots start to come out the drain hole you will need to either move them into nursery grade 3 gal pots and sink them into the main pot (3 should fit) or plant them directly into the large pot. I've done this with large pots before (3 gal pots). I thought it worked out great.

I would just find a place to store those big pots until your minis are big enough to go into them, maybe next spring. In the mean time put the minis in the 1 gallon pots and baby them along so they'll grow nice big root balls!

You could plant the mini in the middle as the center piece of attention (in your 1 gallon pot), and then plant other plants around it. Since you couldn't need the full vertical size of the planter, then be sure to put 1/2 or 3/4 of it filled with some light stuff to where you don't need to fill up the planter entirely with potting mix. For example, you could use styrofoam or plastic containers.

I would say don't plant multiple roses no matter how big the pot is. Yes planting in pots and then putting them in the bigger pot is a fine idea but also make sure that the space that the smaller pots (3 gallon ones) will make in the bigger pot could invite insects bugs and spiders inside.