For the first couple of sessions, I had planned on being learning sessions. After that, I'm not sure. Do you have any suggestions on how to plan for a group like this?
Unfortunately I don't have painted minis yet. I am just getting into the game myself.
What I'd recommend would be setting the players up in control of a unit involved in a larger planetary campaign. There's fighting across a very broad front.
Come up with a roster (that just happens to match your available minis) and then come up with manageable small skirmishes set against the campaign's backdrop. Each session is a different mission, and the outcome can affect the course of the larger battle.
You can find a PDF product in the Catalyst store called "The Mercenary's Handbook." It's an old FASA product, but it still has good content, and even better, it has a useful "Quick Campaign Resolution" ruleset in the back. In between each of your in-store sessions, roll on the table with modifiers based on how well your players did. If they piled it in, negative modifiers for their side; positive modifiers if they succeeded. Then share with the players how their side is doing, and let that drive the narrative for the next mission.
There are a number of "Touring the Stars" products that have printable color planetary maps, so you could even use those to chart the course of the campaign, noting the loss or gain of key strategic sites as it progresses.
By having a larger pool to draw from (the players being just part of a larger unit), you aren't hamstrung if someone can't make it, and have the flexibility to incorporate new drop ins and walkups. Customize the OpFor each time on a ratio based on the player force strength. If the players' side is on a winning streak in the campaign, perhaps lower the enemy ratio. If the players' side has been losing, increase the enemy ratio.
If you want your players to fight each other, then both sides can track the campaign, charting progress against each other as rival units repeatedly skirmish along the line of contact. Setting up such a framework gives the sessions an ongoing narrative where the players want to return to see their side press on to victory, rather than just blowing up enemy pieces in a meaningless battle with no context.