They are average AAA players, that is who will come up when you finish drafting your first 25. It could be that you find such players and if you do you are welcome to them. They do not add to or count against the $70 million salary cap, anymore than AAA players do in an OL. Their official value is 200K.
But they must conform to the PA, IP restrictions and the SB and HR/9 limits. Otherwise, whoever you find is ok. The idea is that these would be precisely people you might want to use to substitute for drafted starters who are under-performing after two-thirds of the season. Most players will be in a platoon system most likely, but whatever strategy you can work out is fine.
This is not that un-realistic. Willie Mays got called up during a season. Derek Jeter played because Tony Fernandez did a Wally Pipp impression in 1996.
As to the theme, it is not low paid players. It is needing to use your whole team - all 25 players, plus a few more. That the players in some cases are very good happens in real baseball.
To give an idea, the two pitchers I selected are people who were stars in other seasons, and have 170 IP total between them, with ERCs (non-normalized) at around 3.30.
I got a second baseman from a WS team with 400+ PA and a .250 batting average, one OF with over 400 PA and a rougly .270 BA and around 15 HRs, and a 3B with a .230 average, again 400 PA and 10-15 HRs. So I don't think we are getting Babe Ruth 1929 here. I don't see a problem. These players are maybe just not quite as good as most of my starting players, but if some of those don't come through, one or more of these guys might find themselves called up and starting.