Forms 2.0 : Tracking a Submit button as an Event in GA Universal

Just did this with a client the other day.  Add your _ga calls inside the onSuccess handler as described in the docs.

So my code should look like this? 

<script src="//app-ab08.marketo.com/js/forms2/js/forms2.js"></script>
<form id="mktoForm_1007"></form>
<script>
MktoForms2.loadForm("//app-sjo.marketo.com", "288-VQZ-957", 1007, function(form){
form.onSuccess(function(values, followUpUrl){
form.getFormElem().hide();
ga('send', 'event', 'button', 'submit', 'infokit');
$('.message-marketo').show();
return false;});
});
</script>

If so, it doesn't seem to fire but I'm not sure if the order of the calls is the problem here.

I would assume you are getting some errors in the Dev Console (rather than it silently failing).  Take a look at those.