That is a rather interesting idea you have set up there! My concern is the complications you may encounter. FFIV, unlike FFVI has no graphic tools other than the generic (and for me, difficult to use) YYCHR TileMolester, those sort of tools, thankfully Chillyfeez has several tutorials in regards to replacing monster graphics and portraits and changing the battle and field sprites is not too difficult of a matter if you know where all of the palettes are stored and the like.
For now I suggest you use placeholder graphics like Rydia for Sonia and the like.
Now FFIV is quite difficult to make into a single party experience for several reasons...
Statuses - In FFIV there are 4 bad statuses that takes the character out of the battle for a time. FFIV did have Gradual Petrification though which has always worked fine against actual party members which would work well for characters like Basilisk, Medusa Heads, etc.
Limited abilities - A Single Character has a lot of trouble keeping up against multiple monsters and it may soon become a bit of an annoyance.
Back Row - One of FFIV's most egregious faults is the Back Row, put a party member back there and your defense is increased by x2 and enemy's hit rate cut in half. This may not sound like a lot until you realize how defense is calculated in FFIV, Say I have 16 Defense with 2 Defense Multipliers and an enemy hurts 40 with 3 multipliers, in a normal sense it would deal 40-16 dealing 24 + 40-16 = 24 + 40 = 84 Damage but say that you're in the backrow, suddenly the enemy's hit rate is now /2 and their damage cut in half...
So, 20-16(50% Chance) + 20-16(50% Chance) + 20(50% Chance) = 0-28 Damage, this makes it always desirable to be in the backrow if alone, no matter what. A hack to get rid of the benefits of backrow are a fairly easy matter.
These are a few of the downfalls of the one character approach. I honestly think that you can have Sonia start alone for a portion and then have her meet Alucard (possibly even have several stages where the two are both in the castle and the point of view will switch in between stages before they've met one another) since all of these problems mentioned above only grow in intensity as a normal game would progress, but it can be considered manageable for several parts.