My toilet is at the end of my drainage line do I have to install the toilet on a tee instead of a 90 and run the vent or can I just add a tee at the main stack before my shower and sink

If it's allowed in your area you can use the sink vent to WET VENT the toilet... and the shower for that matter.

Check your local plumbing codes and see if wet venting is allowed. If it is, and you haven't run the drain pipes yet then you can save a lot of time and money and still accomplish your goals with much less work than running individual vents for everything.

Let me know...

MARK