Ok, are these embedded subtitles, or those downloaded from within Infuse?
Infuse will be limited with certain subtile types, but when using text based (SRT, TXT, OpenSubtitles, etc…) you will be able to adjust and change them however you like.
The options for modifying graphic based subs like PGS are a bit limited. If you were to download subs via Infuse from OpenSubtitles you will have more control over appearance and position.