How can I change the playlists/songs on my phone? I have 43GB of music in iTunes Match on my iMac, and only 5GB available on my iPhone. (8750 songs, 51 Playlists)

Before iTunes Match I could select/deselect playlists then sync and thus change the playlists on my phone. 
Now with Match, if I delete songs within a playlist from the Phone they disappear, no way to reload them. If I delete a playlist on the phone it deletes from iMac/iTunes, thats not good either.

So how can I rotate the songs on my phone without deleting from the iMac/iTunes or losing songs in any way?
You say you have iTunes Match? What I understand from your question is that you want to edit your playlists adding/deleting or you just want to pick specific playlists to have on your iPhone, I'll explain both. 

If you deleted a song from your playlist simply scroll up or down till you see edit/clear/delete. Hit edit then the + icon to add songs, or readd them if you deleted them. Clear will delete all the songs in that playlist without deleting the playlist itself. Delete will delete all the songs and the playlist itself.

If you're hoping to be able to add a couple of playlists for example a" work out" playlist and a "commute" playlist on your phone but leave the "dance" playlist on your computer, you can't. The whole idea of iTunes Match is that you have ALL of your music, playlist and whatnot. The idea is that when you make any change even if it's just moving a song from the bottom to the top of your playlist, it will show up on all your devices. 

I hope that helps let me know if it doesn't and I'll try and help out if I can!

Danny