Thanks for the advice and the welcome, I didnt know such a forum existed!! How handy!
On the topic of boots, would it be ok to wear combination sporting boots on all four legs?
YES :) but not on th back legs.
You do not wear bell boots on back feet. Bell boots are meant for front legs to protect your horse's heels so they do not clip them and cause damage to their heels when they over reach like many barrel horses do. When they do clip themselves then it can cause lameness.
But if I was you I would just buy a set of boots what ever brand you like and get the bell boots seperate. Boots, if taken care off, will last a good while. Where as bell boots are replaced more often, I do not know where you are located but if you can get your hands on some Justin Bell Boots they are the best in my opinion, they last the longest and hold up great. :)
I still have a pair that I bought back in 2009, well I bought two pair a set of black ones and a set of turquoise ones and I have one of each pair still, funny the things you misplace :) but they still serve their purpose faded - yes but I only use them around the barn on what ever "trainee" gets lucky I have others I use for shows.
As for a question to if your horse is too old, I would say Yes and No.
It all depends on the horse, as long as she has no issues (under saddle) and responds to you and is willing to learn then there should be no reason she can not try :) of course any barrel horse has to be in good health as well. My mare is 13 and I am still running her and she has only been doing it for about 2 years now. Also I was running a 20 yr old horse and my friend is still running her 21 yr old to this day, I have also heard of older being run, but mind you they were RUNNING not STARTING. But like I said so long as she is healthy and willing there is no reason your horse can not do the pattern, never say never! :)