We only bathed our old ratty once a month or less, when he really smelled. It became more important once he got older and less inclined to groom himself properly. Some rats enjoy it, some don't, but its not something I would push for if he had got distressed by it.
With the guineas, we towel rub them, hairdryer to waft them then put them in a box stuffed with hay to finish off. If they are inside, we are ok, if they are outside they need washed early evening to make sure they're dry.
juicy we sometimes do a 'lick and a promise wash' with unscented baby wipes (especially if my fluffy little boy has run through a puddle of his brothers wee ) I use them to clear GP1 greasespot too (GP2 doesn't get this trouble)
When we get greasespots turning manky, they need a bumdip.
I never bathed my rats until they got older and less able, one had hind leg degeneration and one respiratory infection so needed a bit help. As they weren't used to being in sink or bath I used a warm wet flannel or unscented baby wipes to reduce stress. If they are elderly males and not grooming correctly it's an idea to gently check for penile plugs I also found using fabric hammocks useful as they absorb the wee and can be chucked in the washing machine as required