Hi,
1. The generic configuration is the generic configuration. It means that it is the same for every payment plugin.
2. You can change the size of the iframe if you want to and let the default value if you don't want to customize it.
3. You can use several payment plugin in the same time if you want. It is really as you wish.
If you want to keep the classical paypal plugin, keep it. It will work.
4. No
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.