Depends on what you mean by coexist. I think they brought Maclin in to play on the outside which was Perriman's expected role. Smith only played 13 snaps in week 5, so I'm going to throw that one out and only look at the 13 games in which he was playing. Extrapolating those 13 games to 16 games, Smith would've had 121 targets, Perriman 59, and Aiken 43. So yeah, if Baltimore goes from a 4 man WR rotation to a 3 man rotation then there will be plenty of targets for that 3rd guy. However, even though Aiken got fewer targets, he actually played 23% more snaps than Perriman which indicates to me that there's something about Perriman's game that the coaches don't like. Personally, I wouldn't be surprised to see Campanaro step in and fill Aiken's role, leaving Perriman with scraps once again. It seems like the team likes Campanaro but he was injured last year.
As for Pitta's targets, I was rooting for the guy, but I'm not sure he can't be replaced with Gillmore and Watson (and Maxx Williams if he's still a thing). It's reasonable to expect some % of his targets to be reallocated to the slot WR, I suppose. Maclin is a capable slot guy, so I'd expect him to move in when Perriman is on the field. If they pay Decker decent money then I'd expect them to play more 3 wide, but no use in speculating unless he actually signs.
But for Maclin, since this is his thread, I think Smith's 7.5 targets per game are his, so if he's going to get 120 targets this year, he's vastly undervalued right now. I expect a similar 120 for Wallace and maybe 65 for Perriman, 50 for Campanaro (or another slot WR to be named later), and 100 to Pitta's TEBC replacement.