Assuming the reverse is true that wattage output will be halved as resistance doubles, would a pair of 15 ohm Rogers LS3/5As (82.5dB/W/M) be too much of a load for a 50 wpc Forte 1A?You're correct- the amp is considered a 'voltage source' so it will put out the same voltage into any impedance (within limits of course) so if you double the load impedance the output power is cut in half.
With ~25 wpc, should I be concerned about clipping?
This isn't a bad thing- into higher impedances any amplifier will make less distortion and you will hear this as greater detail and smoother sound overall. 25 watts into a set of Rogers is a decent amount of power. The speaker is meant for nearfield listening and so can't make a lot of sound pressure. You'll find that in a bedroom system this is plenty of power. I run an am with only 5 watts/channel in my bedroom system and my speakers are only slightly more efficient than the Rogers; I never come anywhere close to overdriving the amp (a sub really helps)!