These transformers will work great for tesla coils.

8.5kV is more than enough.

I built my first conventional coil around a 5500V, 350mA
>I can disconnect the inner ground connections of the 
>serie coils but I am afraid of stressing the isolation to much?
I blew a transformer by not earthing the inner - Keep the 
existing earth connection.

You will need to current limit the primary.

I used a domestic 3kW electric heater initially.
Chokes are better because they don't waste power.
(I used the primarys of micro wave oven transformers with
the secondaries shorted out.Details on my web page see 
"My Conventional Coil")

Start out with a 1-2kW coil - get experience.
A jacobs ladder is a good fun way of checking your current 

One of these transformers will give you 6kW (above 3kW
you need a rotary gap.)
>Is it usefull to serie them with isolated cores?
You want to start at 12kW....
You can put the secondaries in series -connect
the earthed ends of the secondaries together keep that earth 
connection and check the phasing.

have fun,

Alan Sharp (UK)