What muscle groups should I workout together?

Your workout look pretty good but if you just wanted a change in routine you could consider this.

Chest/triceps as you currently do.

Legs/shoulders

Back/biceps because the back exercise involve pulling and therefore the biceps.

I assume that you don't do the same exercises everytime you do a particular body part. For example, don't do bench press for the chest everytime. Do it for a few weeks and then change it to something else like flys or inverted bench press. The body gets used to a routine and bneeds to be shocked every so often.

I also assume that you know higher reps tone and define (12 - 15 reps), whereas lower reps build muscle (6 - 10 reps).

everyone has there own tips and tricks that they swear by try different things and you will find your own. for me i do one part each day

sun cardio

mon triceps

tue biceps/ forearms

wed back

thur hamstrings

fri shoulders

sat quads and calves

it just really depends on what you want to do, for me i want to make sure that every fiber gets broken down and also with that way i have 6 days to fully recover these muscle groups.

if you want definition i would focus on

chest/tri's

biceps/back

shoulders

legs

that way your grouping together pushing and pulling if you want to keep it that way but like i said before find what works best for you and make it your own.