The bulb will only draw what it needs, so a 21W indicator bulb will be drawing less than a 55W fog light bulb. As for the bulb longevity, a 21W indicator bulb is just the same as a rear fog light bulb, and that's happy to stay on all the time. They are as cheap as chips to replace anyway.
Personally tho', I think what you are planning to do will look odd. To those that don't know, i.e. almost everyone else on the road, it'll just loo like your indicators are stuck on. I think US marker lights in the rub strips would be a much better job.