In the house we just purchased the garage is only lit by 5 single light 100 watt incandescent fixtures.
Based on the layout of the exsiting fixtures I can do two 8ft T5ho fixtures running the length of the garage above the main 2 door bay(24'wx27'Lx10'h) and one 8ft over the single garage bay(12Wx29'Lx10'h). The other two lights would either 4ft or 8ft fixtures depending on the light output of the first three that I install.
Unfortunately, your post raises more questions than it answers...
Do I gather that these are two separate spaces, divided by an interior wall or similar? Or is it one big open space, which just happens to have two unequal-size overhead doors serving it?
How are the interiors of these spaces finished? Drywall? OSB? Open framing? Something else?
Is there some particular reason you want/need to use the old screw-in fixtures as your power source for the new lighting?
How are these existing fixtures switched? Are they all wired as one large bank of lights? Separate switches for each one? Something in-between? Also, do you have switches at every conceivably useful location (such as near each entrance door, etc.)?
Be aware that the output from any light fixtures you install more-or-less in the center of a garage bay ceiling will be largely blocked by the vehicle itself, instead of going where you really NEED it when working on that car/truck/whatever.
Which brings us to...
What sort of work do you plan to do in these spaces, and just how much light do (you think) you need to properly support that work?
The garage is insulated but not heated. We have the option for propane heat but it's not installed.
Do you mean, "not installed YET", or that you don't plan to install some sort of heating in the near (or at least "forseeable") future? I've never been to Montana (which is a pity... I hear it's beautiful); but I'd wager that it gets pretty da*n cold there in the Winter. Bear in mind, this is not just a matter of your personal comfort while working in the garage. If you can reliably protect that area from ever seeing freezing temperatures, that makes life MUCH easier on anything and everything you store there -- not only the vehicles themselves, but also any paints, solvents, lubricants, cleaning supplies, etc.
Am I going to run into issues installing the T5ho fixtures? Any other options using the existing incandescent bases?
Again, I'm not sure what the hang-up is about WRT reusing the old fixtures. To my mind, if it's worth going to the work and expense needed to upgrade the lighting in the first place, then one should not let such trivial concerns potentially compromise the final outcome. Select and install the lighting you really want, and which will best do the job you need done, regardless of HOW that winds up getting installed.
I updated my profile. I'm in Montana. Whats the best way to connect the CFL strips to the existing ceiling box?
"CFL strips"?!?
I think at least one of us is confused.
Also what bulbs are recommended? HD has the Phillips 4100k for $80.00 for a case of 15
FWIW, I'm generally not nearly as large a fan of the T5HO type tubes as some here apparently are. They have their place, of course (such as some "High Bay" applications -- which your 10-foot ceilings preclude anyway); but they're mostly about sucking as many lumens as possible out of a relatively small source (such as a single multi-tube fixture), rather than maximizing overall lighting efficiency throughout the space. Typically, what is REALLY needed is better light distribution, as opposed to "brighter" light at the source.