1980 Bajaj Barnfind

I switched all of my bulbs (6v) to LED. And replaced the flasher unit for the turn signals with a modern one as the original one wasn't compatible. Also put a larger battery, more amps, BUT when the turn signals run, the tail light flickers - goes dimmer. Is that normal, or is it faulty light bulb?

Also, now I have a spare battery, a smaller one - is it a good idea to add it to the system by hooking both batteries in parallel?

And the last question - once I replaced the headlight bulb to LED, the hi/lo pilot light stays on all the time? How is that possible?