All the conditions that were necessary for Wudhu to be correct are also necessary for Ghusl, except for Muwalat (not leaving a gap between the actions) - and also, it is not necessary that the body be washed from top to bottom.
If many Ghusls become Wajib on a person, it is allowed to perform all of them in one Ghusl, but one has to specify a Niyyat for each one.
One who has performed GhuslJanabat must not perform Wudhu for Salat. However, one can not offer the Salat without also performing Wudhu if any other Ghusl was done.
In GhuslIrtimasi, the complete body must be Tahir. However, for Ghusl Tartibi, it is not necessary that the complete body be Tahir. Therefore, if before each part of the body is given a Ghusl, that area is washed, then it will be sufficient.
GhuslJabirah is performed just as WudhuJabirah.
If someone is performing a Wajib fast, then he is not allowed to perform GhuslIrtimasi, since that person fasting is not permitted to place his complete head under water. However, if due to forgetfulness, one performs GhuslIrtimasi, his Ghusl will be correct.
It is not necessary that the complete body be washed with the hands during the Ghusl, therefore, in the event that with the Niyyat of performing a Ghusl, the water reaches to all parts of the body, this will be sufficient.