It isn't 8*1.5 for rechargeables. It's 8*1.2=9.6 volts. Unless the charger has current limiting built in, rather than just having a transformer that goes bang when too much is asked of it, it will need something extra to limit the charge current.
The cells will charge to more than 1.2 volts, and can approach 1.5, but can also be damaged by charging at excessive current for too long.
Lead Acid requires a supply with limited voltage as well as limited current. Nickel cells require limited current, but usually at a different value to that required by lead.
My RX charger consists of a £1.99 (back than) battery eliminator with a 50Ω several watt resistor inserted in one leg of the lead and covered in heatshrink. As the voltage of the charging battery approaches that of the supply, the current drops.
Incidentally, to ensure charging of a 12 volt SLA, the charger will need to put out rather more voltage. The 12 volt SLA will likely charge up to about 14 and a bit volts, the charger, to ensure that it can reach this, will need to offer a volt or two more.