How many?
For the sake of fariness, 1 minion per PC.
Level?
For sake of ease (on you), same level as the average PC level (or one level lower you feel it's too high... too much lower and they simply won't be able to hit to the enemies that the PCs are fighting). It can easily be explained as 'only the heartiest of army are coming with the PCs'
XP?
As a general suggestion, for the XP cut, they should get some. You can use the same basic forumla that enemy minions have. 4 minions of the same level count as one creature of that level. So if you have a group of 5 PCs and 4 minions, all at level 3, then I'd count that as a group of 6 (5= regular plus (4 minions divided by 4 ) 3rd level PCs. So when you divide up XP for that example group, you'd divide the XP as if there were 6 people (rather than just the 5 PCs).
Control?
If you're doing 1 minion per PC to determine the number, then let each PC control 1 minion. But as you give them control you clarify the caveat "they are still technically NPCs so you retain DM veto power if you think they should be doing something else" (this plays a factor in your morale question)
Morale?
No need to make rolls -- I found in previous editions that morale rolls were difficult given the ever changing situations and the subsequent requirement to determine how much to modify on a constant basis. Just keep it a judgment call. The first order that sounds suicidal, sure maybe they trust them. But once that first guy goes down they may hesitate when asked to charge to the corner of the field all by themself with no backup, etc.
Or, if your *players* are easily motivated by greed, you can just dangle a carrot "don't treat them as cannon fodder, there will be a reward down the line for each minion left alive by the end of it". The reward need not be explicitly as a result of the minions srviving, but down the road toss in an extra treasure equal to X * Y where X is the number of minions left alive and Y is some gp amount that you determine. OR you can also just give the PCs the XP value of the minion for each one that is still alive at the end. But don't tell them what the reward will be (otherwise they will debate how vaulable the reward is), just dangle the carrot.