Yep it will be back a rookie that can evolve into it again, but since these rookies can evolve into another mons, you can go to a diferent evolution patch. This give lots of flexibility to get the mons that you want since you don't need to farm the shards for the same mon all the times you awake it.
The WarGreys, one of then is the starter lv40 MGrey? If yes, it is your priority to use as fodder to awaken another mon, but only do that when you get more megas. The Terra Force ones, choose the best one as main digi to use on farm on dailies or exp/friendship on normal quests.
You don't need to farm 74 shards from the same mega that you want to be +4, only the 21 final ones. The 53 shards can be distributed into diferent megas - of course you need to plan a lil' to make sure that you will have as option said mega, but the site that I passed and ingame chart helps a lot with that.
Don't give up on any digi, including the starter ones, even if they can't have max number of chips, they can digivolve to mega and be used as fodder for the digis that you want to get +1/+2/+3/+4.
And don't forget to keep upgrading your meat farm, warehouse and restaurant - on the last one, spend most stamina you can before tap/click on it to finish the construction, since it recovers all your stamina.