Now that I have a Birdsnest & a Monti, I want to make sure they can survive...
Should have done that the other way, make sure they can survive then get them
But a birdsnest is a fairly easy to keep coral, so it's a good one to start with to see if they can be kept alive. The montipora, depending upon the species can be a bit more forgiving then some acropora species, so if anything could be a good set of starter corals to see if your light is bright enough.
From what you said, I guess having full blue LED + 10k T8 might help?
Maybe... not sure if that T8 bulb is a high output bulb, but a general rule of thumb for tube lights, the larger they are the less bright they are (for any given length). IMO, if you have the LEDs that should be your growing lights there, I'd toss an actinic bulb on the T8s so you can get some fluorescence, and if you have it on a separate light timer many like to lead in the day with the actinic light and end the day with actinic light just so they get that time period in the morning/evening where the tank is dark but fluorescence can be seen.
Also I might need advise in switching to this light, cos I've come across posts where SPS get bleached when u move to brighter lighting?
My advice, don't put a 48" light on a 36" tank, it's not worth it. You'll be wasting 25% of the light that's not sitting over it, and it really would be an eye sore and a half. Take this from someone who has had my fair share of "hey it works it doesn't need to look good" setups, there are some things that shouldn't be done.
If you're really think of increasing the lighting my advice would be to ditch the T8 bulb, and replace it with more LEDs. And yeah you can bleach SPS with too much light, however it's doubtful you'll get too much light over that tank unless you toss on a 400W MH bulb right over the surface. What you can get is light shock by having corals adjusted to a particular light and then vastly increasing the amount of light. So very often you simply want to go in baby steps. If you have dimmable LEDs that can easily be done by slowly ramping up the intensity over a few weeks, for non-dimmable lights you can use a series of screens to dim things up.