A lot of it depends on your personnel and those you are absorbing. Now, I've never been a mercenary, but I have been in management in a high turnover environment, and a lot of it can come down to who is being promoted and who expects to be.
So, if you have a core unit, say a company, and a fair few of your warriors are quite ready to be lance commanders, then that could breed discontent, since they don't really get to shine, since in a company unit there's no real upward mobility. But, if you can import four extra warriors, then all of a sudden your best warrior can be a lance leader, and perhaps you can make your best lance commander leader of a two lance unit. So long as the newcomers understand that they're the newcomers and they respect that hierarchy, no problem, because they have old timers all around them, and your people see a chance to grow themselves. Then you can do four more, or eight, and so on.
But, in your second scenario... I suppose it still depends. If that lance has a decade in the field with a great record and every member has the skills and leadership abilities, then bringing in more warriors to give each warrior their own lance would very likely work. But, if you started having new officers over yours, I can see resentment, unless the new officers were so impressive as that their worth couldn't be ignored (ex-elite house guards or A+ merc, for example). While if you kept your officers for your people and let the newcomers have theirs, then you don't integrate well and that can cause cliquishness and problems.
Though I'd ask if you're gaining three times your numbers, who's absorbing and who's being absorbed?