Glass carriers with a condenser enlarger like a 23C mean more dust to spot out from your prints.
Glass carriers are a bit less of a dust problem if you have a colour or variable contrast diffusion light source.
Glass carriers keep the negative much flatter - that will matter particularly if you do large prints.
A 6x6 glassless carrier will work with 6x4.5 negatives, but make sure you leave your negatives in strips that are at least three negatives long - preferably four or five long.
If you print a lot of 6x4.5 negatives with a glassless carrier, you will probably end up wanting a 6x4.5 carrier eventually, because that carrier will allow you to have the negatives centred, even if they are at the end of the strip.
In the meantime, If you have a 6x6 glassless carrier, you can make lots of prints, so go for it.