As for the question about fat-loss (sorry I missed it before, i accidentally skimmed over it). The only thing I can say is that femara has been used in studies to treat obesity. Since serum testosterone can be somewhat lower in obese males due to the slowed synthesis of LH and FSH production, letro has been shown to normalize these serum test levels, as we all know LH and FSH increase with the use of letro and testosterone obviously aids in weight loss we can assume that letro can be used to help with weightloss. However, testosterone levels are only diminished in cases of fairly severe obesity. For most people just looking to lean up more on this site I somewhat question the ability for letro to do the trick so to speak.
If you do want to give this a try I see no problem with doing it off cycle, the normal dose in studies of obesity and testosterone profiles was again 2.5mg per day.
Anyways, to finish off I would honestly say you would be better off using an ECA stack or clen (even though Im not a big fan), over letro. If you are on cycle you could try using letro even if you dont need an AI to see if there is any difference but I would say again running test (usually at a minimum dose of 500mg per week) the impact of letro on weight loss will be slight to say the least, imo you wont be able to notice a difference.