I think MLB sticks to 13 total pitchers and usage adapts to that. If shorter starts are part of the equation then you have 1 of 2 scenarios:
5 starters - assume 2 go 6-7 innings per outing (Nola and Eflin for example). Other 3 only go 4-5.
8 relievers - 3 designated as long men with at least 1 LHP. They are the tandem pitchers for the short starters and mop up if necessary. 3 back end relievers (say Neris, Morgan, Neshek now). 2 more middle guys for the 6th and 7th inning when your tandem fails or your 6 inning starter does not get there.
This only works if you can get your 3 bullpen long men into the 80-100 inning range because you want your other relievers still closer to 60 innings.
Scenario 2 - 4 starters, 80 pitches. 4 tandem relievers go with them and also pitch 60-80 pitches. LHP and RHP pairs encouraged among your 8 starters.
5 more relievers. 3 back end, 2 middle as before.
Tandem starters as a rule only really work with a shorter rotation. Would ML teams be bold enough to try this? Tampa might. It is hard to do so if you have any starters like Snell capable of doing more, but even under a 4-man rotation you could make sure Snell gets the 5th inning even if others only make it 3 or 4.
If this happened MLB would clearly need to change the 5-inning win rule for pitchers. Never made sense to me that a starter could go 4.2 and a reliever could get one out and a win anyway. Whoever is pitching when you take the lead should get the win.