Hi, I know you consider the color icons closed, but I will still reply as other users might find it useful.
Those last 3 action buttons come from plugins, so you should follow a similar approach in the individual plugin .css files (/var/www/html/fop2/admin/plugins/xxxx/xxxx.css
The main action buttons are the ones I listed, it seems you have mixed versions of icon images, you can open them individually and check their size, there is no reason to have one of the correct size and not the others if they share .css data.
In any case, it is just html/css, nothing else/nothing more. It is not that difficult to alter/modify and make whatever you like with styles and visual representations, always abiding to html/css limitations/constraints.
As for changing the extension to something else, you must modify the extension header on your configuration. When using automatic configuration you cannot change that via FOP2 manager, but you can change that manipulating the FOP2 buttons table. Suppose you want to try with extension 100, and if I understand correctly, you want transfers to be directed to*89100 instead of just 100. In that case do this
mysql -u root -p asterisk -e "UPDATE fop2buttons SET extension='*89100' where extension='100'"
After updating the field, restart FOP2:
service fop2 restart
And try to transfer to that extension... if it works and does what you want, then you can do the same for the rest of your extensions.