I know he's being considered for 15 but what are the chances Kellaway gets thrown into 13? He's played there quite a bit for the Rebels and is a really good defender, passer and communicator. Could fill a lot of the roles that Ikitau plays. Also means Petaia can play at 15 which gives is a big boot and some serious aerial ability along with Nawaqanitawse in our back three. Just a thought so please don't @ me.
13 is a tough position to cover as a utility. When I've seen him there for Rebels i've felt like he's doing a job but it's not ideal - you are comprimising him by not playing him in his best position (the physicality of test midfield probably doesnt play to his strengths) and you are comprimising the team by not playing a centre at centre. Of course for the Rebels it's often been a case of him at 13 out of need as there has been limited other options.
Ideally you'd have Kellaway on the wing as that's where he's looked most comfortable but certainly more suited to fullback than centre in my opinion.
Petaia would provide the size you want at 13 but Perese has the advantage of having mostly played centre the last few years. Foketi might lack some pace for 13 - especially up against Rieko. Perese 13, Petaia bench Kellaway 15 makes sense for Melbourne I think.