I love doing musician photos!
1) is good pretty spot on frame/pose. The light gloss off the cello bothers me. Top right corner?
2) no. I wouldn't put an instrument or anything for that matter, pointing at the face/head like that. It's one of the basic rules for posing musicians. His face needs to be cleaned up hair wise.
3) could work for a small headshot for bio work or whatever, but would probably need to be b+w for that sort of lighting. I like the crop.
4) nice work.
5) not enough strobe light for outdoors. detail- where is his end pin? the end pin is usually expected to be extended and supporting the cello from the bottom. IMO, a cello player would notice this and assume he has really strong thigh muscles. Not natural at all. the hard things with musician portraits is they have to be natural, even if you're working with a long tube of metal, a giant piece of old wood, or a small dinky clarinet. Looks like a fun portrait, but I wouldn't do it because I like to either crop close to top half for musician photos, or they have to show full post like the 1st picture. The cello needs to rest on the ground, and I like to have the person look well grounded as well. (alexander techinque). technically, the horizon is not lined up as well
Kinda like this example of old Macy's Ad.

IMAGE LINK: http://s46.photobucket.com …59_n_zps4bd33afb.jpg.html