I'm not an expert , however , some rule of thumbs..
all batteries must be in fairly the same shape,meaning that you best not mix batteries that have not been used in similar conditions , sometimes hard to tell , if they come out of another machine and are the same brand you could more or less be sure they are in a similar condition , do not mix brands
all batteries must have the same voltage , first measure all batteries voltage, if they are not off to much best thing to do next is to parallel them so they 'charge' one another until all voltage are -almost- the same,
again , only do this if the batteries are a bit off (e.g. 1 volt , but that's a guess)
then it should be fairly save to use a trickle or other charger but do keep in mind polarities and voltages, I've seen batteries exploding long time ago , not a pretty sight.
last one, never use a car charger on deep cycle batteries , this will fry the palatines inside and in worst case literally blow up the battery housing, the case will bulk .
best regards,
Luc
Thanks for the explanation Luc