Simple flashing led questions.

The limit for connecting LEDs in series with a flashing LED is determined by the maximum voltage for the flashing LED and the Vf of the other LEDs at the current the flashing LED passes when "off".

I'm sure that sounds like gobbledygook, but it serves to illustrate that the question is not trivial.

With specs for the flashing LED and the other LEDs you are using, it could be calculated, but it depends a lot on the individual components too, so there would likely be a fairly wide spread between what was definitely safe to what was probably safe.

It may be far easier to use the flashing LED to turn on a transistor which, in turn controls further strings of LEDs.

But if you're going that far, it isn't much harder to use a 555 with the associated greater control over frequency and duty cycle.

The best place is in series with the battery. It doesn't matter if it's in series with the positive or negative lead. By convention, it's usually placed in series with the positive lead.

You would normally use insulated wire. For the current you require for LEDs, almost anything is OK. Generally, I wouldn't use something so fine that it was difficult to solder, and mechanically weak. Something with a total size of wire similar to the LED lead would be suitable. Generally you'd use multi-stranded wire.