Right - stop worrying :)
10.0 refers to 10% UVB output. That's overkill for them, for sure, as I'd use that for desert dwellers, not turtles from relatively temperate climates. It's nothing to worry about
now, though, as an old tube won't be pumping out anything like that much UVB. Like I said - go for the 5.0 tube.
Yes, you need a heat lamp too - fluorescent tubes are not basking lamps because they give out very little heat. An ordinary 60W bulb (or 100W if your house gets cold) is all I use for basking lamps. They won't choose to bask on the platform until there's a nice warm basking lamp for them above it. They're probably climbing out to rest but quickly getting cold and diving back in. You'll need to provide that overhead heat somehow, as soon as possible, but personally I've never had to work out how to do that with a tank hood present...
It's much easier to have an open topped environment with the tubular and basking lamps suspended at the appropriate height, above.
Of course you have the issue that the fluorescent light fitting is fixed into the lid at the moment...and you wouldn't want a basking lamp under that hood as it could get wet and would be too close to the platform, but when you replace their tank (and boy, yes, they do look very cramped!) then you can do it all without a lid.
Any option for cutting out a large section of the lid, above the platform, for now?
I got your pics. It may still be a bit early to tell for sure, but I'm 95% certain that you sent me pics of a girl turtle 