Poppy has a great suggestion, nothing you can loss.. just try it out! Overhead cast probably OK for this line... but not spey cast...It depends some on the rod but if you are going to overhead cast I think that line will work. Put it on there and give it a try. It will take a few minutes and you'll know one way or the other.
Yes, that Costal Express might be a little light and it might be a little short but since it's all the OP has it will do the job as he has found out.Can't buy a spey line for my needs (surf/kayak fishing) until I get some funds from selling an extra setup.
Bender's suggestion is a good one.so you can use T-11 to lenghten it and make it heavier.
Max length for OH casts for fishing I recommend is 65Ft but it is useful only when it is almost calm and it delivers without struggle max 4 inch slim streamer but it allows so long casts that handling the shooting line becomes the limiting matter.what's the max head length you'd recommend? I'd rather get a pre-made shooting head with a welded loop rather than trying to custom make one.
20ft T16 too much if added to the end of my coastal express or just fine?The thing about Southern Cali surf fishing is that it actually involves "SURF"
Poppy at Red Shed probably sells the newer 30' lengths of T14 that come looped at both ends from the factory, loop that to an intermediate sinking running line, or use trilene big game #40 mono for running line if you are broke. normal floating spey heads WILL NOT WORK for Southern California surf fishing. You do not want any floating aspect to your running line, head, tips nada, anything that floats will get picked up by the waves and will be deposited at your feet every time.
30 feet of T14 weighs 420gr should work no problem on a 15' 10wt
when you get some money buy a standard fast sinking Rio Outbound 12,
honestly I prefer 30' of T14.
If you need any help or want to fish sometime PM me.