I have used the grey liquid sealer that are sold by the mfg of speed bleeders for many years on many bikes. I do not have speed bleeders on this bike. If this sealant was going to react with brake fluid, I don't believe they would sell it specifically for the threads on speed bleeders. And it seals the threads well enough that enough pressure can be applied to depress the spring and open the ball valve, and fluid does not leak around the threads.
Also, yellow teflon tape is specifically made not to react with gas, propane, etc, etc...you can read the label. There is a chance that if one used too much tape and too far down on the threads that small pieces could tear and be introduced into the system, but caution in application would prevent this.
On the front brakes, is there enough gravity that when removing the bleeder on the caliper that fluid will all drain out?
A plug in hand is a good idea, but how quickly might the fluid drain out, if it does at all???