This has been tested. Enemy force composition is determined solely by the mission, with no relation to the force you select.
Further expansion on this.
If you save your file after the mission loads into the battlescape, the enemy composition is always going to be the same. It's now set. However, if you save before the mission and drop again - even with the same units - it randomizes the OpFor again. (I do not know if this also randomizes who drops in on a three-party mission where there's a third force poking in.
I have not tested this.)
A minor second detail. The rough location of OpFor Lances is always the same for a mission/biome combination which uses the same map area. The maps for each biome are actually rather huge and sliced off into sections for missions to drop you into. If the name of the mission contract, and the biome listed, are the same you SHOULD theoretically be dropping into the same landscape... and in that case the OpFor locations are set to appear in certain areas. Reinforcements which appear will always appear in the same general locations.
(Be careful with that last detail - you do
not want to immediately pick up sensor contact with them. Trying to set an ambush for them by standing right where they spawn inmeans they'll go first thanks to the odd way the Initiative determines it. Later-designed missions might have a scripted DropShip landing, and those are absolutely hazardous to your 'MechWarriors' health.)